This invention relates to restricted off-axis field optical systems and, more particularly, to such a system having a large spectral range. Optical systems constructed in accordance with the concepts of this invention are particularly adapted, among many other possible uses, for effecting the exposure of photoresist-coated semi-conductor wafers in the manufacture of integrated circuits.
The present invention is related to the optical system disclosed by R. M. Scott in U.S. Pat. No. 3,821,763, and to the projection system disclosed in my U.S. Pat. No. 3,748,015 both of which are assigned to the assignee of the instant application, and the disclosures of which are incorporated by reference herein. The Scott patent discloses a restricted off-axis field optical system in which an annular slit is used to restrict the field to an annular zone centered on the optical axis and the system optically corrected to preferentially increase image quality in the annular zone. My prior patent discloses a catoptric, off-axis annular field optical system for forming in accurate micro detail an image of an object at unit magnification with high resolution including convex and concave mirrors in face-to-face relationship with their centers of curvature being substantially concentric. The mirrors are arranged to produce at least three reflections within the system, and they are used in the system with their axial conjugates at unit magnification in planes normal to the axis of the system, the axis containing the centers of curvature. While this optical system has many features and advantages, the present invention is directed to improvements thereover, which will become apparent as the description proceeds.
Other related patents in this field include Russian Pat. No. 126,911, issued Mar. 30, 1959; French Pat. No. 784,063, issued July 22, 1935; U.S. Pat. No. 3,244,073, issued Apr. 5, 1966; U.S. Pat. No. 3,821,763, issued June 28, 1974; U.S. Pat. No. 3,951,546 issued Apr. 20, 1976 and U.S. Pat. No. 4,011,011, issued Mar. 8, 1977, which is a Continuation of U.S. patent application Ser. No. 339,860 filed Mar. 9, 1973 (now abandoned); "Achievements in Optics" by A. Bouwers, Elsevier Publishing Company, Inc., 1946, (particularly pages 24, 25 and 39); and "New Catadioptric Meniscus Systems" by D. D. Maksutov, Journal of The Optical Society of America, Vol. 34, No. 5, May 1944, pp 270-284.
It is noted that throughout the following specification and claims statements made referring to an annular field of mean radius H also refer to a restricted off-axis field at a distance H from the axis. The terms color or chromatic variation as used herein is intended in its extended sense, i.e., it includes variation with wavelength in any region and is not restricted to visible light.